When working bores so that they will have an accurate size, that is, an accurate internal dimension, it is difficult to maintain a desired geometry. Typically, such a bore may be a cylinder of a cylinder-piston combination. It is difficult to provide a honing tool which will be vertical throughout the entire travel or length of the bore, so that the bore will be accurately cylindrical. Frequently, the bottom of the bore, that is, the longest feed movement of the honing tool, and particularly if the bore is a blind bore, will be slightly narrower than the open end. This is due to the construction of honing tools which, particularly in blind bores, cannot provide for overrun of the tool.
Many solutions have been proposed to generate accurately dimensioned cylindrical bores; none of them are entirely satisfactory or lend themselves to automatic procedures.
It has been proposed to so construct honing tool carriers that the honing blades or cutters are placed on movable strips. The movable strips are then so arranged that the portion of the strips which will engage the lower part or the end of the blind bore is subjected to a greater degree of operating pressure than the portion of the strip closer to the open end of the bore. In spite of the lack of overrun of honing tools in blind bores, that is, complete passage of the tool through the bore, cylindrical bores can be generated with a high degree of accuracy. The honing tools, however, since they will effectively have greater pressure at the lower end, that is, at the inner end of the blind bore, require use of conical tool elements, and the honing cutters or blades or strips must be relatively short. If the blind bore is shallow, for example when making a brake drum, satisfactory results can be achieved. When making long blind bores, however, for example cyliners of internal combustion engines or the like, two sequential working steps may be used: A first tool having short parallel-arranged honing strips is introduced into the bore with short feed strokes to pre-work an enlargement at the lower or inner end portion of the bore. Thereafter, a second tool with longer parallel honing strips is introduced for final working of the bore to final dimension throughout its length. Such an arrangement requires a double-spindle honing machine, or change of the honing head or tool head on a single-spindle machine.
It has been proposed to utilize a tool which has a set of longer honing strips which can be spread outwardly, to desired dimension, parallel to the axis of the bore, and which additionally has a set of shorter honing strips which can be spread apart either in conical form or parallel to the axis of the bore. The adjustment of the longer as well as the shorter honing strips is independent from each other. Such a tool permits a method of operation in which the longer honing strips are continuously used, the shorter ones, however, are subjected to operating pressure, for example hydraulic pressure, only when the honing tool has reached the bottom of the bore; upon withdrawal of the honing tool, pressure is released. Blind bores of excellent cylindrical character can be made, with the degree of accuracy of the cylinder wall with respect to the axis of the cylinder, especially in the region of the bottom of the bore, depending essentially on feed, workpiece material, applied pressure, and duration of operation, and on the feed path of the longer honing strips which are arranged parallel to the axis of the bore.
Adjustment of the respective operating values, that is, overall feed of the honing tool, pressure of the longer parallel strips, and pressure of the short strips, has to be emperically determined. Adjustment of the apparatus, and operating the honing machine, is done manually, particularly during an initial honing step. It is frequently necessary to correct the adjustment and positioning of the respective honing strips upon first introducing the tool into the bore. This requires a high degree of skill on part of the operator; extensive adjustment and fitting and set-up time are required, and the eventual accuracy depends greatly on the experience and skill of the operator.
It has previously been proposed to utilize an automatic control when honing through-bores by checking the dimension of the bore hole and controlling the overrun of a honing tool through the through-bore in dependence on errors between the actual dimension of the hole which is being honed and a desired or command dimension. It has also been proposed to automatically control honing of blind bores by delaying or retarding the feed of a honing tool and, rather than controlling the overall travel path of a honing tool, to control only the feed of the tool when it reaches the base of the blind bore. Such an apparatus does not operate satisfactorily, however, since, when working blind bores, and particularly those which have an essentially flat bottom, or a very limited central depression or cone tip, honing tools which have only a set of honing strips capable of parallel adjustments will still hone cylinders which are off-size, i.e. slightly narrower in the region of the bottom of the blind bore. The decrease in feed rate which results in multiple and finer material removal is not sufficient and has not been found capable of compensating for the lack of overrun of the entire tool which is possible only through an open through-bore.